To reinforce what's been stated, the important elements spawn provide are that they're amazing bullet sponges when in cover, they move very quickly without being slowed by terrain, and they can do some damage in close combat (though aren't particularly amazing). They synergize very well with a glass hammer IC joined to the squad to deliver AP2 / AP3 swings while they take the brunt of the retribution. Independently, they're ok but not amazing.

Obliterators are a Swiss army knife unit. They're not as cost-efficient as the alternative options for the HS slot (predators are 3 lascannons for 140 points, oblits would be 225 for 3 lascannons every other turn), but their multi-purpose role, moderate maneuverability, and decent survivability (particularly with MoNurgle) can justify paying a premium - especially for a limited number of them.

In short, both units have a good supporting role in limited quantities - obliterators because their cost gets very restrictive in large quantities, and spawn because they're reliant on another choice (HQ) to make them particularly scary instead of an annoying tar pit. Though for a few list compositions, I can see walls of spawn working great as just delay tactics for the workhorse units behind them.